From Osmotic Stress to Antibiotic Toxicity: Salinity as a Key Modulator of Enrofloxacin Effects in the Manila Clam ( Ruditapes philippinarum )

ABSTRACT

Antibiotic residues, particularly fluoroquinolones, increasingly co‐occur with salinity fluctuations in coastal areas, yet their combined effects on marine bivalves remain poorly understood. This study evaluated the interactive impact of enrofloxacin (ENR; 5 and 500 ng/L) and salinity (30 and 40 PSU) on the marine bivalve Ruditapes philippinarum , using an integrative biomarker approach to assess metabolic, oxidative, and cellular responses. Salinity emerged as the dominant factor influencing physiological patterns, as shown by multivariate analysis, with proline levels, electron transport system (ETS) activity, and lipid peroxidation (LPO) driving treatment differentiation. Exposure to high salinity alone was associated with osmotic adjustment through proline accumulation, enhanced metabolic activity, and maintenance of lipid reserves. In contrast, under combined stress (ENR at 40 PSU), clams exhibited reduced proline content, depletion of lipid reserves, and decreased ETS activity, indicating metabolic depression and reallocation of energy reserves. Under combined exposure (ENR at 40 PSU), proline accumulation decreased, lipids were depleted, and ETS activity declined, indicating metabolic depression and mobilization of energy reserves. LPO levels were lower under dual stress than under high salinity condition alone, suggesting a reduced mitochondrial reactive oxygen species production. Cellular energy allocation (CEA) further reflected increased energetic costs under combined conditions. These findings demonstrate that salinity acts as a key modulator of antibiotic toxicity, producing non‐linear and context‐dependent responses. This highlights the need to incorporate environmental variability into ecotoxicological assessments, particularly in light of climate‐driven changes in coastal salinity regimes.